1. Field of the Invention
The present invention broadly relates to generally to a scanner provided with two-axis tilting of the scanning beam reflective member which may be used in reading holographic data. The present invention further broadly relates to such a scanner having an assembly which provides the scanner with position feedback on the tilt axis orientation of the reflective member.
2. Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, holographic memory systems have been suggested as alternatives to conventional memory devices. Holographic memory systems may be designed to record data as one bit of information (i.e., bit-wise data storage). See McLeod et al. “Micro-Holographic Multi-Layer Optical Disk Data Storage,” International Symposium on Optical Memory and Optical Data Storage (July 2005). Holographic memory systems may also be designed to record an array of data that may be a 1-dimensional linear array (i.e., a 1×N array, where N is the number linear data bits), or a 2-dimensional array commonly referred to as a “page-wise” memory system. Page-wise memory systems may involve the storage and readout of an entire two-dimensional representation, e.g., a page of data. Typically, recording light passes through a two-dimensional array of low and high transparency areas representing data, and the system stores, in three dimensions, the pages of data holographically as patterns of varying refractive index imprinted into a storage medium. See Psaltis et al., “Holographic Memories,” Scientific American, November 1995, where holographic systems are discussed generally, including page-wise memory systems.
Holographic data storage systems may perform a data write (also referred to as a data record or data store operation, simply “write” operation herein) by combining two coherent light beams, such as laser beams, at a particular point within the storage medium. Specifically, a data-encoded light beam may be combined with a reference light beam to create an interference pattern in the holographic storage medium. The pattern created by the interference of the data beam and the reference beam forms a hologram which may then be recorded in the holographic medium. If the data-bearing beam is encoded by passing the data beam through, for example, a spatial light modulator (SLM), the hologram(s) may be recorded in the holographic medium.
Holographically-stored data may then be retrieved from the holographic data storage system by performing a read (or reconstruction) of the stored data. The read operation may be performed by projecting a reconstruction or probe beam into the storage medium at the same angle, wavelength, phase, position, etc., as the reference beam used to record the data, or compensated equivalents thereof. The hologram and the reconstruction or probe beam interact to reconstruct the data beam.
A technique for increasing data storage capacity is by multiplexing holograms. Multiplexing holograms involves storing multiple holograms in the holographic storage medium, often in the same volume or nearly the same volume of the medium. Multiplexing may carried out by varying an angle, wavelength, phase code, etc., in recording and then later reading out the recorded holograms. Many of these methods rely on a holographic phenomenon known as the Bragg effect to separate the holograms even though they are physically located within the same volume of media. Other multiplexing methods such as shift and, to some extent correlation, use the Bragg effect and relative motion of the media and input laser beams to overlap multiple holograms in the same volume of the media.
In angle multiplexing, multiple holograms may be stored in the same volume of the holographic storage medium by varying the angle of the reference beam during recording. For example, data pages may be recorded in the holographic storage medium at many angles, the exhausting the dynamic range or “address space” of a given volume of the medium. Each location in the “address space” (or each data page) corresponds to the angle of a reference beam. During recording, the reference beam scans through many discrete angles as data pages are written. Conversely, during readout, a conjugate reference beam (sometimes referred to as a “probe beam”) may probe each data page at its corresponding angle. The scanner may be used for either recording or readout.